The specific absorption rate (SAR) is a measure of a rate at which a human body absorbs energy when exposed to a radio frequency (RF) field. RF fields are created by many common devices, including mobile phones, smartphones, cellular phones and so on. The specific absorption rate measures the power absorbed per mass of tissue, and is typically measured in watts per kilogram (W/kg). The SAR may be averaged over the whole body, or over a small sample volume, such as 1 g of tissue.
Various regulatory bodies, such as the Federal Communications Commission (FCC) in the United States, and the European Committee for Electrotechnical Standardization (CENELEC) in the European Union, establish specific absorption rate limits for exposure to RF energy close to the human head. For example, the FCC requires that any mobile phones have a SAR level at or below 1.6 W/kg over a mass of 1 g of tissue. Similarly, CENELEC requires that mobile phones have a SAR level at or below 2 W/kg over a mass of 10 g of tissue.
The SAR of a device is typically tested using an anthropomorphic mannequin head that simulates a human head. Testing is performed by placing the device at various positions on both sides of the phantom head and measuring the SAR at each position. Further, testing is typically performed at the maximum transmit power of the device under test. For devices that do not transmit continuously, SAR testing averages duty cycles to determine the SAR of the device.
From a communications standpoint (and not considering battery life), it is typically advantageous to use more rather than less transmit power in order to successfully transmit information to a remotely located receiver. However, health concerns have resulted in regulatory standards that limit transmit power in handheld communication devices such that human exposure to RF radiation stays within the specified limits.
What is needed are methods and apparatus for reducing the amount of transmit power that is absorbed by humans using such devices in close proximity to themselves.